There are conventional vehicular lamps that use a semiconductor light-emitting element such as a light-emitting diode (LED) as a light source. The amount of luminescence of the LED when used as a light source is less than that of a light bulb. Therefore, such conventional vehicular lamps employ a configuration that connects multiple LEDs in parallel.
In a lamp configured to use multiple LEDs connected in parallel as a light source, even if some of the LEDs do not emit light as the result of an abnormality such as a disconnection, the other LEDs can still emit light so as to satisfy a light distribution standard required for a lamp (e.g., a law prescribing light source luminosity). If the number of abnormal LEDs increases, however, it becomes less likely that the light emission of the normal LEDs can satisfy the light distribution standard required for the lamp.
Hence, a vehicular lamp has been proposed that includes a disconnection circuit for detecting an abnormality of the LED, especially disconnection of the LED (see, e.g., Japanese Patent Application Laid-Open (Kokai) No. 2006-248509 (pages 4 to 6 and FIGS. 2 and 3)).
The vehicular lamp described in the foregoing patent document is equipped with a luminous unit that is composed of the parallel connection of multiple LEDs; a lamp that is fitted with a line breakage detection circuit designed to detect line breakage of individual LEDs in the luminous portion; and a judgment circuit that is linked with the line breakage detection circuit through a signal line and designed to judge abnormality of the LEDs. The judgment circuit is configured so as to determine an abnormal LED based on an electric potential of the signal line.
In the foregoing vehicular lamp, when an individual LED becomes disconnected, an electric potential corresponding to the disconnected LED is provided from the line breakage detection circuit to the judgment circuit via the signal line. Therefore, the judgment circuit can determine the number of abnormal LEDs and their location/arrangements within the lamp based on the electric potential of the signal line.
However, in the foregoing vehicular lamp, line breakage detection circuits are provided for each LED connected in parallel. This increases the number of components needed to build a structure for satisfying the light distribution standard required for a lamp using the line breakage detection circuits.